F-Strings

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Python also includes another method of building strings, which are known as “F-strings”. F-strings allow us to put placeholders in strings that are later replaced with values from variables, effectively creating a way to build “templates” that can be used throughout our program.

The easiest way to see how this works is looking at a few examples. Let’s start with a simple one:

name = input("Enter your name: ")
print(f"Hello {name}")

If the user inputs "Willie Wildcat" when prompted, then this code will produce this output:

Enter your name: Willie Wildcat
Hello Willie Wildcat

There are many important parts to this example, so let’s look at each one individually. First, in the print() statement, we see the string "Hello {name}". This is an example of a template string, which includes a set of curly braces {} as placeholders for data to be inserted. Each template string can have unlimited sets of curly braces. Inside of each set of curly braces, we can place the variable or expression that will be substituted at that location as a string.

Also, we notice that in front of the string, we see the character f. Preceding a string with f outside of the quotation marks will denote the string as an f-string (hence the name), which allows the values inside to be interpolated. Interpolation is the term used when formatting marks in a string, such as the curly braces in an f-string, are interpreted and replaced with the correct values they represent.

Formatting Numbers

The most powerful use of f-strings is to insert numerical values directly into strings without having to convert each value directly to the str data type - the interpolation process handles this for us.

For example, we can update our previous program to use f-strings to display the output in a single print() statement, and we can also add additional information with ease:

text_one = input("Enter the price of one item: ")
price = float(text_one)
text_two = input("Enter the quantity of items: ")
quantity = int(text_two)
cost = price * quantity
print(f"{quantity} items at ${price} each is ${cost} total")

When we execute this program, we’ll see output that looks like this:

Enter the price of one item: 2.75
Enter the quantity of items: 3
3 items at $2.75 each is $8.25 total

This example shows how easy it is to build complex output strings using f-strings. We can expand the template syntax to even round numbers. f"{.2f}" allows us to format a number to two decimal places. The 2 represents the number of places you want to round to and the ‘f’ inside the curly brackets indicates that you are trying to format the number as a floating point number.

text_one = input("Enter Numerator:")
numerator = float(text_one)
text_one = input("Enter Denominator:")
denominator = float(text_one)
result = numerator / denominator
print(f"{numerator} divided by {denominator}, rounded to two decimal places is {result:.2f}")

Enter Numerator: 2
Enter Numerator: 3
2.0 divided by 3.0, rounded to two decimal places is 0.67

If we add a comma into the template string f"{,.2f}" , we can automatically format in commas for thousands, millions, etc. For example:

text_one = input("Enter the price of one item: ")
price = float(text_one)
text_two = input("Enter the quantity of items: ")
quantity = int(text_two)
cost = price * quantity
print(f"{quantity} items at ${price:,.2f} each is ${cost:,.2f} total")

300 items at $2,500.45 each is $750,135.00 total

print("{} items at ${} each is ${} total".format(quantity, price, cost))